tests: drop redundant and slow `random` test

Suggested by Emil [1]:
> Feel free to drop the random test altogether. It's an old public API
> no active users (pretty ancient code uses it) and the in-tree users
> drmSL and drmHash already have respective tests.

This test takes minutes to run, while all the other tests combined take
barely more than a second.

Dropping it also helps the CI by avoiding random timeouts when `random`
takes more than the 4 minutes (!) we've allowed for it.

[1] https://gitlab.freedesktop.org/mesa/drm/merge_requests/26#note_390066



Suggested-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>